Decarbonylative cyclization

The net effect is decarbonylation, cyclization and stereoisomerization which are linked to the structure of the carbohydrate moiety. The main results describe carbohydrates bearing a carbonyl group on the aglycone (Norrish I) or on the ring (Norrish II). 

Both direct and acetophenone-sensitized irradiation of the diene lactones (168) have been carried out. In the direct irradiation, three reactions occur in competition namely decarbonylation, cyclization, and the formation of the bicycloheptenones (169). Triplet sensitized irradiation yields the bicycloheptenones (169) exclusively. The cyclization of (168c) to (169c) is in competition with phenyl migration processes. ... 

Gold-catalysed cycloisomerization of l,7-diyn-3,6-bis(propargyl carbonate)s provide for a diastereoselective synthesis of naphtho[ ]cyclobutenes by a cascade sequence involving gold-catalysed double [3,3]-rearrangement, 6 r-electrocyclic reaction, and a decarbonylative cyclization (Scheme 15). °... 

Scheme 34 Ru-catalyzed decarbonylative cyclization of terminal alkynals to cycloalkenes and cycloisomerization to cyclic a,p-unsaturated aldehydes...

Scheme 35 Ru-catalyzed decarbonylative cyclization of 1,6-diynes to ej o-alkylidenecyclopentanes...

EPR spectroscopy is usually used to calibrate the clock (i.e., to determine kc). The method described here uses EPR to detect the two radicals. These are the parent (R1 ) and the product (R2 ) of its reaction, be it cyclization, decarbonylation, decarboxylation, rearrangement, or whatever. The radical R1 is produced photochemi-cally in the desired inert solvent by steady and usually quite intense light irradiation of the EPR cavity. Typically, R1 and R2 attain steady-state concentrations of 10-8 to 10 6 M. 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

An example of the 7d-type cyclization is that of iminium salt 38b obtained by decarbonylation of a-(ferf-amino) acid 38a. Attack of the nucleophilic malonate on the newly formed electrophilic carbon of the iminium ion gives dicarboxylate 39a (79JOC4173). 

The hydrative cyclization involves the formation of a ruthenium vinylidene, an anti-Markovnikov addition of vater, and cyclization ofan acylmetal species onto the alkene. Although the cyclization may occur through a hydroacylation [32] (path A) or Michael addition [33] (path B), the requirement for an electron- vithdra ving substituent on the alkene and lack of aldehyde formation indicate the latter path vay to be the more likely mechanism. Notably, acylruthenium complex under vent no decarbonylation in this instance. 

Indolo[3,2-3]carbazoles bearing a carbonyl substituent at the 6-position have been synthesized directly by an intramolecular cyclization approach from dichloroacetyl substituted precursors <2004EJO2593>. However, due to a facile decarbonylation under the reaction conditions it was found to be more expedient to employ esters, such as 165, produced by acylation with oxalyl chloride (Equation (79) Table 4). 

The cadmium secocorrinoid carboxylic acid (102 M = Cd) also undergoes photocyclization to the acid (103 M = Cd), which on transmetallation to the nickel(II) complex (103 M = Ni) and treatment with triethylamine and acetic acid yields the parent corrin complex (100 M = Ni).268 The decarboxylation process is extremely facile. A related base-catalyzed cyclization of the secocorrinoid aldehyde (104) gives the corrin complex (105), which can be decarbonylated to the parent complex (100 M = Ni) by treatment with potassium hydroxide (Scheme 66).268... 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

Free-radical cyclization of phenyl selenide 15 to indolizidinone 16 represented a key step in the total synthesis of (—)-slaframine (equation 52). The two pairs of diastereomers were first separated and then hydrolyzed to the corresponding alcohols in 76% overall yield77. (TMS)3SiH-mediated acyl radical reactions from phenylseleno esters 17 have recently been utilized for the stereoselective synthesis of cyclic ethers78. In fact, the experimental conditions reported in equation 53 are particularly good for both improving cis diastereoselectivity and suppressing decarbonylation. 

Terminal alkynals (113) of appropriate length (n = 1, 2) and substitution [X = C(C02-Me)2, C(CH2OR)2, NTs, and others] have been cyclized with decarbonylation to cycloalkenes (114), using a ruthenium(I) catalyst.348 In some cases, cycloisomerization to give conjugated aldehyde occurred. Both processes are believed to involve catalytic ruthenium vinylidenes. 

Christie, B. D. Rapoport, H. Synthesis of optically pure pipecolates from L-asparagine. Application to the total synthesis of (+)-apovinc-amine through amino acid decarbonylation and iminium ion cyclization. J. Org. Chem. 1985, 50, 1239-1246. 

The utility of the solid-state photodecarbonylation of crystalline ketones was recently demonstrated in the total syntheses of two natural products, where the key step is the solid-state reaction. The first example involves the synthesis of the sesquiterpene ( )-herbertenolide [80] by the solid-state decarbonylation of cyclohexanone 189, followed by cyclization of the photoproduct 190 (Scheme 2.46). With precursor 189 obtained by simple methods and a solid-state reaction carried out to 76% conversion, herbertenolide was obtained in good overall yield in a record number of steps from commercial starting materials. With a similar synthetic strategy, samples of the natural product (i)-a-cuparenone were obtained in about 60% overall yield from 191 by a very succinct procedure that included four simple steps and a solid-state reaction at — 20 °C [81]. 

This cyclization can be used to convert the aldehyde 6 into the bicyclic ketone 7, an intermediate for the synthesis of carbacyelin (8). In this case, decarbonylation competes with cyclization, and the yield is only moderate, ... 

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